1. Field of the Invention
The invention relates to image intensifier tubes of the type wherein, firstly, an incident ionizing radiation is converted into photons in the visible or near-visible range and wherein, secondly, a slab comprising microchannels is used to ensure a gain in electrons.
2. Description of the Prior Art
Image intensifier tubes such as these are often called "proximity focusing" tubes. They are used, for example, in radiology. The principle of radiological image intensifier tubes ( IIR tubes in short ) using slabs of microchannels is well known. It is described notably by J. Adams in "Advances in Electronics and Electron Physics", volume 22A, pp. 139-153, Academic Press, 1966.
FIG. 1 gives a schematic view of the structure of a standard IIR tube using a slab of microchannels such as this.
The IIR tube 1 comprises a vacuum-tight chamber, constituted by a tube body 2 positioned about a longitudinal axis 13 of the tube. The body 2 is closed at one end by an input window 3 and at the other end by an output window 14.
The X-rays penetrate the IIR tube through the input window, which should be as transparent as possible to these rays: the input window 3 is generally constituted by a thin metal foil (aluminium, tantalum, etc.).
The X-rays then encounter a layer 4 of scintillating material in which they are absorbed and give rise to a local emission of light proportional to the quantity of X-radiation absorbed. The scintillator material may be, for example, caesium iodide forming the layer 4 with a thickness of the order of 0.1 to 0.8 nm. The layer 4 of scintillator material is supported by a single plate 5 transparent to X-rays, formed for example by a thin metal foil (for example made of aluminium alloy) or else a silica-based glass plate etc. The supporting plate 5 is located towards the input window.
The scintillator 4 bears a photocathode 6. The photocathode 6 is constituted by a very small thickness (often smaller than one micrometer) of a photo-emissive material. This layer is deposited on a face of the scintillator 4 that is opposite the supporting plate 5. The photocathode 6 absorbs the light emitted by the scintillator 4 and, in response, sends out electrons locally into the surrounding vacuum, in proportion to this light. The set constituted by the supporting plate 5 bearing the scintillator 4 which itself bears the photocathode 6 constitutes a primary screen 15.
The electrons (not shown ) emitted by the photocathode 6 are directed by an electrical field towards the input face 8 of a slab 7 of microchannels. To this effect, a first potential and a second potential V1, V2 are applied respectively to the photocathode 6 and to the input face 8, the second potential V2 being more positive than the first potential V1.
The slab 7 of microchannels is an assembly of a multitude of small parallel channels 12 assembled in the form of a rigid plate. Each primary electron (sent out by the photocathode) that penetrates a channel is multiplied by a phenomenon of secondary emission in cascade on the walls of the channel, so that the flow of electrons at the output of the slab can be more than a thousand times greater than the input flow. The diameter d1 of the channels may range from 10 to 100 micrometers. The channels 12 are inclined with respect to the normal to the plane of the slab so that the electrons which are emitted by the photocathode 6 in parallel to this normal cannot emerge from a channel without giving rise to a phenomenon of secondary emission. In order to reduce the number of electrons that strike the input face of the slab 7 outside the channels 12, it is the usual practice to make a widened portion 35 at the input to these channels and hence to reduce the thickness of their walls. The thickness E of the plate that forms the slab 7 of microchannels is typically between 1 and 5 mm. The electronic gain of the slab may be adjusted over a wide range of values, for example between 1 and 5000, as a function of the voltage developed between the input face 8 and an output face 9 of this slab 7, namely an output face 9 to which a third potential V3 is applied.
The electrons at output of the slab of microchannels are accelerated and focused by an electrical field, on a luminescent screen (10) positioned so as to be facing the slab, parallel to this slab, and at a distance D of the order of 1 to 5 mm. The luminescent screen 10 locally emits a quantity of light proportional to the incident electron current. The luminescent screen therefore restores a visible and intensified image of the X-ray image projected on the scintillator, through the input window of the tube. The luminescent screen, which is a layer with a thickness of some microns, constituted by grains of luminophor material, is deposited on a glass port which may constitute the output window 14 of the tube. The face of the luminescent screen 10, pointed towards the slab 7 of microchannels, is coated with a very thin metal layer 18, made of alumininum for example. The metallization enables the electrical polarization of the screen (by the application of a fourth potential V4 that is more positive than the third potential V3) and acts as a reflector for the light reflected rearwards by this screen. The port 14 supporting the screen 10 may be made of glass, or may be constituted for example by a fiber-optic system. The screen 10 may be deposited directly on this port or on an intermediate transparent support if it is desired to insulate the screen 10 from the port because of constraints of use.
The primary screen 15 and the slab 7 of microchannels are fixedly joined to the body 2 of the tube, for example by means of lugs 21, 22, 23 sealed to this body. To these lugs, there are furthermore applied the polarizing potentials V1, V2, V3. The polarizing of the input and output faces 8, 9 is furthermore ensured by means of a metallization (not shown) with which, as a rule, these input and output faces of the slab are generally coated except, naturally, in positions facing the channels 12. The primary screen 15 and the slab 7 are thus fixed so as to be electrically insulated from each other while, at the same time, being separated by a relatively small distance D1 of the order of some tens of millimeters (it must be noted that for greater clarity, FIG. 1 has not been drawn to scale).
These conditions are necessary to obtain, between the photocathode 6 and the input face 8 of the slab, an electrical field suited to the task of accelerating the electrons emitted by the photocathode 6 towards the input of the microchannels of the slab 7; this electrical field should be intense enough to limit the angular dispersion of the electrons which tends to reduce the spatial dispersion of the IIR tube.
Furthermore, the distance D1 between the photocathode 6 and the slab 7 should be maintained uniformly to obtain high image resolution on the entire field.
Under these conditions, the accurate positioning of the primary screen 15 and, especially, of the photocathode 6 with respect to the slab 7, is a lengthy and delicate operation that is made even more difficult by the low mechanical rigidity of the supporting plate 5 (bearing the scintillator 4) in order to absorb the X-radiation to the minimum extent.
An additional complexity is provided by a difference between the expansion coefficients of the scintillator 4 and of its support 5. The result of this difference is that the primary screen 15 structure tends to get deformed, and that it is difficult to limit this deformation to less than some tens of millimeters when it takes effect over lengths close to several centimenters. Furthermore, if the primary screen 15 is moved away from the slab 7 to minimize the influence of the deformations, the result is an unacceptable loss of resolution.
Now, what is sought is the industrial-scale manufacture of IIR tubes with proximity focusing, capable of picking up large-sized images as is the case with IIR tubes in which the image, formed on the output screen by the electrons emitted by the photocathode, results from a focusing of these electrons by means of an electronic optical device. In IIR tubes using electronic optical devices, the primary screen may commonly attain a diameter of up to about 50 centimeters.
It is clear that, with such dimensions, the positioning of a primary screen with respect to a slab of microchannels raises serious problems. At present, this constitutes one of the major drawbacks of IIR tubes with proximity focusing. However, this type of tube has advantages as compared with those using an electronic optical device. Thus, for example, this type of tube may be much flatter than the latter type of tube (with a smaller distance between the primary screen and the output screen); furthermore, it can be made more easily to receive and form a rectangular image.